1. FIELD OF THE INVENTION:
The present invention relates to a heat exchanger used in an air-conditioner, a refrigerator or the like and adapted to transmit heat between fluids.
2. DESCRIPTION OF THE PRIOR ART:
As shown in FIG. 1, this type of conventional heat exchanger generally includes heat transfer tubes 2 made of copper and the like connected to each other by means of U-bends and fins 1 made of aluminum or the like, and is constructed such that a fluid flowing through the heat transfer tubes 2 and air flowing between the fins 1 effect heat exchange.
In recent years, there has been a demand for such types of heat exchanger to be made compact and to have high performance. However, since the speed of air flow between adjacent fins is suppressed to a low level with a view to reducing noises and for other reasons, the fin surfaces on the air side involves a very much higher heat resistance than the inner surfaces of the tubes does. For this reason, there are taken measures which are to enlarge a fin surface area so as to reduce the difference in heat resistance between the fin surfaces and the inner peripheral side of the tubes. However, there are limitations on such enlargement on the fin surface area, and the heat resistance on the fin surface-side still substantially exceeds that of the inner peripheral side of the tubes.
For this reason, attempts have been made in recent years to work the fin surfaces in such a manner as to reduce the heat resistance between air and the fins. FIGS. 2a and 2b show a conventional example of an improved heat exchanger, FIG. 2a being a top plan view of a flat fin, and FIG. 2b being a cross-sectional view taken along the line IIb--IIb in FIG. 2a. In the drawings, reference numeral 4 denotes heat transfer tubes; 5 fin collars; 6 a fin; 7a-7h cutouts; and 8 an air flow. The multiplicity of cutouts 7a-7h are provided alternately on the front and rear sides of the fin 6 such as to be located between adjacent heat transfer tubes 4 in a vertical direction. In this case, thin temperature boundary layers are produced on the multiplicity of cutouts 7a-7h, respectively, and an improvement in the heat transfer performance can be made by the so-called temperature boundary layer front-edge effect.
However, if the heat transfer capability of local portions of the fin configuration shown in FIGS. 2a and 2b is examined elaborately, the cutouts 7a, 7b on the upstream side of the air flow 8 exhibit a large boundary layer front-edge effect and a high heat transfer capability, while air having been subjected to heat-exchange in advance by the cutouts 7a, 7b on the upstream side of the air flow 8 is not mixed with the other air and flows to the cutouts 7c-7h on the downstream side of the air flow 8. More specifically, since the cutouts 7c-7h lie inside of a temperature boundary layer generated by the cutouts 7a, 7b, the heat transfer performance is not so good. In addition, a dead water zone is produced downstream of the heat transfer tube 4 with respect to the air flow 8, into which zone air does not flow, and in which zone the heat transfer performance is poor. For these reasons, no remarkable improvements have hitherto been found in the heat transfer performance.